Steering equipment for assisting a driver to steer an automobile is well known in the art. In conventional steering assemblies, the operator controls the direction of the vehicle with the aid of a steering wheel. This wheel is mechanically connected, usually through a gear assembly to the road wheels. To aid the operator, many systems utilize an auxiliary system to generate a force that is transmitted to a steering gear assembly. The additional force reduces the effort required by the operator in changing the direction of the vehicle. Typically, this auxiliary force is generated by either a hydraulic drive or an electric motor.
One advantage in having a direct connection is that the operator receives tactile feedback through the steering wheel. For example, if the vehicle changes directions while it is moving, the operator will feel resistance in the steering wheel.
With a steer-by-wire system, since the mechanical link between the steering wheel and the road wheels is eliminated, some potential problems arise when trying to get the system to have the right feel e.g., tactile feedback to the driver. At the same time, since there is no longer a mechanical connection, what the driver feels at the steering wheel is highly tunable. One major issue with the elimination of the mechanical connection is that the phase relationship between the driver""s steering wheel angle input and the torque felt by the driver can change significantly. This change in phase relationship can cause the system to have poor steering feel and may have a destabilizing effect on the system.
A typical steer-by-wire system uses steering wheel position information in order to control the position of the road wheels. Then the forces at the road wheels are measured and used to provide the feedback torque to the driver. This approach results in driver steering wheel position and the resulting torque felt by the driver being largely decoupled. From a steering feel perspective, there is a desirable phase relationship between steering wheel angle and steering wheel torque. This phase relationship is not guaranteed and actually may not even be possible using only feedback of the forces from the road wheels to determine steering wheel torque. There is also a desirable torque magnitude felt by the driver (as a function of input frequency). As the magnitude of this desired torque goes up the chance for instability increases especially once the driver removes his hands. This is due to the fact that the torque provided by the motor to achieve the desired feel is being balanced (in off-center and steady state sense) with the driver""s effort. Once the driver removes his hands, however, the torque provided by the motor accelerates the steering wheel to center and possibly overshoots, depending on the magnitude of the initial torque. As this overshooting action is taking place, the hand wheel system sends the corresponding position signal to the road wheels, and the road wheels return to center. However, due to lack of a driver resistance (and thus a hand wheel overshoot,) the road wheels overshoot, as well. Therefore, the road wheel forces switch direction, and thus, the steering wheel motor switches the direction of its torque (in response to the sensed road wheel forces). This causes the hand wheel to come back to center (from the opposite off-center position now), and an overshoot of center may take place, again. The excessive overshoot and oscillations is known as xe2x80x9cFree Control Oscillationxe2x80x9d. Since these oscillations are due in part to lack of resistance by the driver, it is reasonable to add some kind of resistance in the hand wheel actuator to control this system.
Conventional position control of actuators, servos, and the like often utilize a feedback control system to regulate or track to a desired position. The control law maybe a proportional, integrative or derivative gain on the tracking error or may be a more sophisticated higher-order dynamic. In either case, the feedback measurement is the actual position and in some cases, it""s derivatives.
This mechanism is sufficient for most applications where the load on the system has a predictable relationship to the system position (rotational or translational). In control system terms, this could be predicted by the location of the poles and zeros of the system or frequency response. A conventional control system could then be designed based on these dynamics.
However, in many systems, the load varies based on operating conditions even with the position and its derivatives kept the same. For example in automotive steering applications, the load on the steering system changes as a function of the road surface, operation (lateral acceleration, vehicle speed etc) and tire properties. In such cases, the conventional control design is optimal for a given operating condition, but has reduced performance as the conditions change.
Therefore, is it considered advantageous to provide a steering control system, which addresses the load on the system while still providing the assist forces and tactile feedback for the operator and reducing free control oscillation.
A steer-by-wire control system comprising: a road wheel unit responsive to a road wheel command signal for steering a vehicle, the road wheel unit includes a road wheel position sensor to produce and transmit a road wheel position signal and a road wheel force sensor to produce and transmit a road wheel force signal. The steer-by-wire control system also includes a steering wheel unit responsive to a steering wheel torque command signal, the steering wheel unit includes a steering wheel position sensor to produce and transmit a steering wheel position signal and a torque sensor to produce and transmit a feedback torque sensor signal. The steer-by-wire control system further includes a vehicle speed sensor to produce and transmit a vehicle speed signal, a master control unit operatively connected to the vehicle speed sensor, the steering wheel unit, and the road wheel unit. The road wheel unit is also responsive to the road wheel force signal and/or the steering wheel unit is also responsive to the steering wheel position signal.
A method for steering a vehicle with a steer-by-wire system comprising: receiving a road wheel force signal, a road wheel position signal, a vehicle speed signal, a steering wheel position signal, and a feedback torque sensor signal. The method for steering a vehicle also includes generating a steering wheel torque command signal responsive to the road wheel force signal, the vehicle speed signal, and the feedback torque sensor signal. The method for steering a vehicle further includes generating a road wheel command signal responsive to the road wheel position signal, the vehicle speed signal, the feedback torque sensor signal, and the steering wheel position signal. The method for steering a vehicle further includes generating a force compensated position command signal in a road wheel unit responsive to the road wheel force signal and/or generating a position compensated torque command signal in a steering wheel unit responsive to the steering wheel position signal.
A storage medium encoded with a machine readable computer program code including instructions for causing a computer to implement the method for steering a vehicle with a steer-by-wire system disclosed above.
A computer data signal comprising instructions for causing a computer to implement a method for steering a vehicle with a steer-by-wire system disclosed above.
Also disclosed herein is a position control system responsive to load comprising: a position control unit responsive to a position command signal for controlling position, the position control unit includes a position sensor to produce and transmit a position signal and a sensor to produce and transmit a force signal. Wherein the position control unit comprises a closed loop control system responsive to the position command signal, a position signal from the position sensor, and the force signal.
Also disclosed herein is a torque control system responsive to position comprising: a torque control unit responsive to a torque command signal for controlling torque, the torque control unit includes a position sensor to produce and transmit a position signal and a torque sensor to produce and transmit a torque sensor signal. Wherein the torque control unit comprises a control system responsive to the torque command signal, the torque sensor signal, and the position signal
Further disclosure herein includes a method for controlling position with a position control system responsive to load, the method comprising: receiving a force signal, a position signal from a position sensor, and a position command signal, and generating a force compensated position command signal in a position control unit responsive to the position command signal the force signal and the position signal. Wherein the force compensated position command signal, the position signal, and the force signal are employed in a position control unit forming a closed loop control system.
Further disclosure herein includes a method for controlling torque with a torque control system responsive to position, the method comprising: receiving a position signal from a position sensor, a torque sensor signal, and receiving a torque command signal, and generating a position compensated torque command signal in a torque control unit responsive to the torque command signal, the torque sensor signal, and the position signal. Wherein the position compensated torque command signal the position signal, and the torque signal are employed in a torque control unit; and the position compensated torque command signal, the position signal, and the torque signal are employed in a torque control unit forming a closed loop control system.
A storage medium encoded with a machine readable computer program code including instructions for causing a computer to implement a method for controlling position with a position control system responsive to load.
A storage medium encoded with a machine readable computer program code including instructions for causing a computer to implement a method for controlling torque with a torque control system responsive to position.
A computer data signal, said computer data signal comprising instructions for causing a computer to implement a method for controlling position with a position control system responsive to load.
A computer data signal, said computer data signal comprising instructions for causing a computer to implement a method for controlling torque with a torque control system responsive to position.